Is it true that the plastic foam which provides the cushioning in safety
helmets degrades over time and becomes ineffective ?

If so, is this degradation linear from day one, or does it
begin after a certain period and is the degradation affected
by careful use or is it a matter only of time?

I know that pressure on the foam as in even a minor accident,
renders a helmet ineffective; and that the foam gradually
compresses with use. Is there a scientifically measured stage
at which a helmet should be scrapped? Eg time since purchase;
hours of use? Why is it that helmets which use this foam as their
safety medium do not have a use-by date clearly stated on each helmet?

Timothy Gay, a nuclear physicist at the University of Nebraska, and
who has an interest in sports physics being the author of
the book "Football Physics", responded.

The foam of an (American) football helmet does degrade over time,
but the most important parameter is the number of "cycles" or
compressions the foam undergoes. Factors such as perspiration
and temperature can effect foam lifetime as well. In the United
States, helmet manufacturers recommend reconditioning of the
helmet every other year. The reconditioning procedure may or
may not involve replacement of the helmet's internal foam elements,
depending on their condition.

All reconditioned helmets are drop tested to determine if they
meet NOCSAE (National Operating Committee on standards
for Athletic Equipment) standards. Those that do are identified
with a sticker.

To my knowledge, no studies have been carried out to see
if foam degredation is linear with the number of cycles. Ideally,
one would like to see a graph of maximum head acceleration
for a given drop height vs cycle number.

Also, keep in mind that unlike bicycle helmets, which are
"one use", and rely on the destruction of the outer shell to
absorb energy, football helmets absorb energy by foam
compression, which is quasi-reversible.